Non-covalent intermolecular forces (e.g., electrostatic, hydrogen bonding and van der Waals interactions) play a vital role in many biological processes. Examples of such processes include enzyme catalysis, drug-receptor interaction, antigen-antibody interaction, biotin-avidin interaction, DNA double helix formation, phagocytosis, pigmentation in plants and animals, and cellular transport.
Targeted delivery of medically useful molecules is well known and has been successfully applied in diagnostic and therapeutic applications. In conventional bioconjugate chemistry, almost all of the conjugates are prepared by covalent attachment of various effector molecules, such as drugs, dyes, hormones, magnetic resonance imaging agents, echogenic agents, radiopharmaceuticals, or chemotherapeutic agents, to bioactive carriers, such as antibodies, peptides and peptidomimetics, carbohydrates, or small molecule receptor agonists and antagonists. For example, in diagnostic medicine, various radionuclide and radionuclide chelates covalently attached to antibodies, hormones, peptides, peptidomimetics, and the like have been used to detect lesions such as tumors, inflammation, ischemia, and thrombi (Tenenbaum et al., Radiolabeled somatostatin analog scintigraphy in differentiated thyroid carcinoma, J. Nucl. Med., 1995, 36, 807–810; Haldemann et al., Somatostatin receptor scintigraphy in central nervous system tumors: Role of blood-brain barrier permeability. J. Nucl. Med., 1995, 36, 403–410; Babich et al., Technetium-99m-labeled chemotactic peptides: Comparison with indium-111-labeled white blood cells for localizing acute bacterial infection in the rabbit. J. Nucl. Med., 1993, 34, 2176–2181; Knight et al., Thrombus imaging with technetium-99m-labeled synthetic peptides based upon the binding domain of a monoclonal antibody to activated platelets. J. Nucl. Med., 1994, 35, 282–288). Thus, the target site may be cells such as tumor cells, platelets, erythrocytes, leukocytes, macrophages, vascular endothelial cells, myocardial cells, hepatocytes, etc., or the extracellular matrix surrounding these cells.
In addition to the direct administration of biologically active compounds into the body, molecules such as these have also been encapsulated within organized amphiphilic aggregates such as a liposome, vesicle, or other multilamellar structures. The aggregates are then delivered to the particular organs or tissues of interest (U.S. Pat. Nos. 5,985,852; 5,785,969; and 5,542,935).
A requirement for targeted delivery is a strong interaction between the binding compound or targeting moiety (e.g., ligand) and the target or site of attachment (e.g., receptor) in the formation of a complex (e.g., ligand receptor complex). The dissociation constant (Kd) value of the complex should typically be in the nanomolar range. Compounds exhibiting Kd values from about 100 nM and upward are considered weak binding compounds and are not generally considered to be useful for targeted imaging and therapeutic applications. However, there are many in vivo biological processes that do operate using multiple weak-binding interactions. These include, for example, enzyme-substrate complexes and cell adhesion molecules, which operate in the micromolar binding range. Cell adhesion molecules such as E, P, and L selectins are important biological modulators implicated in inflammatory and thrombolytic processes. (McEver, Selectin-carbohydrate interactions during inflammation and metastasis, Glycoconj. J., 1997, 14(5), 585–91; McEver et al., Leukocyte trafficking mediated by selectin-carbohydrate interactions, J. Biol. Chem, 1995, 270(19), 11025–8; Bischoff, Cell adhesion and angiogenesis. J. Clin. Invest, 1997, 100 (11 Suppl), S37–39; Lesley et al., CD44 in inflammation and metastasis, Glycoconj. J., 1997, 14(5), 611–22; Siegelman et al., Activation and interaction of CD44 and hyaluronin in immunological systems. J. Leukoc. Biol., 1999, 66(2), 315–21).
Previous work in this area involved the use of only high-binding ligands (Torchilin et al., Preservation of antimyosin antibody activity after covalent coupling to liposomes, Biochem. Biophys. Res. Commun., 1979, 89(4), 1114–9; Allen et al., A new strategy for attachment of antibodies to sterically stabilized liposomes resulting in efficient targeting to cancer cells. Biochim. Biophys. Acta, 1995,1237, 99–108; Zalipsky et al., Peptide attachment to extremities of liposomal surface grafted PEG chains: preparation of the long-circulating form of laminin pentapeptide, YIGSR. Bioconjug. Chem., 1995, 6(6), 705–8). Although increased binding was observed in this system, a cooperative effect was not needed because the ligand was already endowed with sufficient affinity for targeting purposes. In contrast, weakly binding ligands presented a formidable challenge.
There is considerable effort to improve the binding affinity of relatively weakly binding selectins and selectin mimics, and to attach them covalently to effector molecules for imaging and therapeutic purposes (Fukuda et al., Peptide mimic of E-selectin ligand inhibits sialyl Lewis X-dependent lung colonization of tumor cells, Cancer Research, 2000, 60, 450–456). However, such efforts usually involve synthesizing and screening large numbers of new chemical entities to discover the ones that exhibit substantially improved binding properties. Furthermore, simple conjugation of effector moieties to an already weakly binding carrier is expected to result in a bioconjugate whose bioactivity is either greatly diminished or obviated altogether. Thus, there is a need for a simple method to enhance the affinity of any weakly binding targeted molecules in order to enhance their usefulness for diagnostic and/or therapeutic purposes.